1. Technical Field
The invention relates to a device for reproducing a digital information signal from a channel signal, the digital information signal being runlength limited with one or more constraints, comprising receiving means for receiving the channel signal.
The invention also relates to a method for reproducing a digital information signal from the channel signal.
2. Related Art
The invention is in the field of channel coding, in particular in runlength limited channel coding. As known, usually the length of time, expressed in channel bits, between consecutive transitions is called the runlength. A (d,k) sequence is characterised by two parameters, a d-constraint and a k-constraint. Hence, this (d,k) sequence satisfies the following two conditions: due to the d-constraint, two logical xe2x80x9conesxe2x80x9d are separated by a run of at least d consecutive xe2x80x9czerosxe2x80x9d; due to the k-constraint two logical xe2x80x9conesxe2x80x9d are separated by a run of at most k consecutive xe2x80x9czerosxe2x80x9d. The (d,k) sequence is converted (from the (d,k) domain) into a runlength-limited (RLL) sequence of the type (d,k) (in the RLL domain) upon precoding in a 1T precoder. This RLL sequence comprises elements with runlengths (either an array of consecutive zeroes or an array of consecutive ones) of d+1 at minimum and k+1 at maximum between subsequent signal reversals in the information signal. The values of (d+1) and (k+1) indicate the minimum and maximum runlengths of the element allowed in the sequence. It must be noted that in stead of the term xe2x80x9can element with a runlengthxe2x80x9d the term xe2x80x9ca runlengthxe2x80x9d is used. It must also be noted that the term element can be used to indicate both an element of a (d,k) sequence and an element of an RLL sequence. An element is considered to be extending over a runlength in the RLL domain or (d,k) domain.
In European Patent Application No. 98201515.8 (PHN 16.906), a non-prepublished patent application, an device being capable of generating a digital information signal, the digital information signal being runlength limited with a (d+n)-constraint and a (k+n)-constraint is described.
In this patent application, a device is described that converts an encoded channel signal with a d-constraint equal to 2 into another channel signal that has a d-constraint equal to 1. This conversion can be carried out in the (d,k) domain or in the RLL domain. As a result, all runlengths will be shortened by one bitcell, resulting in a converted channel encoded signal. Upon channel decoding, the runlengths can be enlarged by adding one bitcell, resulting in a replica of the original signal. If this device is reading out an information carrier and misses one element with a certain runlength the following problem occurs. When deconverting the digital information signal with a d-constraint equal to 1, into a digital information signal with a d-constraint equal to 2, for each missed element (e.g. while reading out an information carrier comprising the digital information signal) two bits should have been inserted into the d=1 constrained channel signal as each element results in two transitions. In other words, missing one element with a certain runlength results in two missing transitions (resulting in a bitslip) in the channel code, which error can not be easily overcome with standard Read-Solomon error-correction measures, like CIRC, the error-correction process used in the Digital Audio Compact Disc. The bitslip errors consequently results in errors up to the next sync word in the signal.
The invention has for its object to provide a device for reproducing a digital information signal, which mitigates the effect of errors present in a channel signal received by the device.
The device in accordance with the invention is characterised in that the device further comprises means for comparing a detected runlength with a predetermined value indicative of a minimum runlength constraint or a maximum runlength constraint of the channel signal and for generating a control signal when the detected runlength violates said constraint and substitute means for in response to the control signal deleting or inserting an element in the channel signal.
The invention is based on the recognition that the runlength of an element of the channel signal can indicate that during reproduction at least one element with a certain runlength has not been detected or that an extra element has been inserted. By comparing the runlength of the channel signal with a value indicative of a k-constraint of this signal, an element of the runlength limited digital information signal can be replaced with another element. In this way the missing runlength can be inserted into the information signal. Doing so, the number of transitions in the signal is reconstructed correctly. This has as a result that, when converting the signal by changing the runlengths by n bitcells, the correct number of bits are inserted or deleted and no bitslip errors occur. Similarly, by comparing the runlength of the channel signal with a value indicative of a d-constraint of this signal an element of the runlength limited digital information signal can be replaced with another element. In this way an extra inserted runlength can be removed from the information signal. This extra insertion can for example occur when a large runlength is detected in a noisy channel, causing the detection of small runlengths. E.g. in a channel signal with a d-constraint of 3 an 17 runlength can erroneously be detected as the runlengths I2-I3-I2.
Another device according to the invention is characterised in that the predetermined value is larger than the maximum runlength in the channel signal.
The presence in the signal of an element with a runlength with a predetermined value which is larger than the maximum runlength in the correctly detected channel signal is an indication that the reproduced information signal contains errors. These errors can be caused by a so-called transition-shift but also by a missing runlength. This device therefore has as an advantage that a substitution of part of the reproduced digital information signal only occurs if a relatively large violation is present in the reproduced signal. If a substitution would take place on each violation, an unjust substitution could take place, e.g. in case a small k-violation caused by a transition-shift would occur. In this case (i.e. an error in the reproduced signal due to a transition-shift) no runlength is missing and therefore no substitution is allowed in order to correct the information signal.
Another device according to the invention is characterised in that the channel signal comprises control elements that exceed the predetermined value and that the predetermined value is larger than the length of control elements present in the channel signal. Another device according to the invention is characterised in that the control elements are synchronisation marks.
The channel signal usually comprises control elements, for example synchronisation marks. These synchronisation marks are intended to synchronise receiving the information of the channel signal when reading out the carrier. In order to be able to detect these control elements in a reliable manner and in order to be able to distinguish these control elements from the other, data containing, elements, these control elements have a runlength violating the k-constraint in a relatively large manner. E.g. a (2,8)-sequence comprises of synchronisation marks with a runlength of I11. This device therefore has as an advantage that a substitution of an element of the channel signal only occurs if a k-violation is present in the reproduced signal which exceeds a xe2x80x9cviolationxe2x80x9d of this constraint due to the detection of a synchronisation mark. If a substitution would take place on a k-violation due to the detection of a synchronisation mark, an unjust substitution would occur, introducing additional errors.
Another device according to the invention is characterised in that the element substituted by the substitute means has a runlength according to the minimum runlength constraint.
Experiments have indicated that, when reading out a record carrier, the small runlengths have the largest probability of not being detected. This device therefore has as an advantage that a substitution of part of the information signal with an element with a minimum allowed runlength, e.g. in a d=2 constrained code this minimum allowed runlength is an I3 runlength, will have the largest probability that a correct substitution is carried out onto the information signal.
A method for reproducing a digital information signal from a channel signal, the digital information signal being runlength limited with one or more constraints, comprises the steps of receiving the channel signal, comparing a detected runlength with predetermined value indicative of a minimum runlength constraint or a maximum runlength constraint of the channel signal, generating a control signal when the detected runlength violates said constraint, and substituting an element of the channel signal in response to the control signal.